The present invention relates to Mobile IP network technology. More specifically, this invention relates to mechanisms for optimizing IP address use in a Mobile IP environment.
Mobile IP is a protocol which allows laptop computers or other mobile computer units (referred to as “Mobile Nodes” herein) to roam between sub-networks and types of network adapters—while maintaining IP connectivity. Without Mobile IP or a related protocol, a Mobile Node would be unable to maintain IP sessions while roaming through various sub-networks. This is because the IP address required for any node to maintain IP sessions is location specific. Each IP address has a field that specifies the particular sub-network on which the node resides. If a user desires to take a computer that is normally attached to one node and roam with it so that it passes through different sub-networks, it cannot use its home base IP address. As a result, a business person traveling across the country cannot merely roam with his or her computer across geographically disparate network segments or wireless nodes while remaining connected over the internet back to their home network. This is not an acceptable state-of-affairs in the age of portable computational devices.
To address this problem, the Mobile IP protocol has been developed and implemented. An implementation of Mobile IP is described in RFC 3344 of the Network Working Group, C. Perkins, Ed., “IP Mobility Support for IPv4,” August 2002. Mobile IP is also described in the text “Mobile IP Unplugged” by J. Solomon, Prentice Hall. Both of these references are incorporated herein by reference in their entireties and for all purposes.
The Mobile IP process in a Mobile IPv4 environment is illustrated in FIG. 1. As shown there, a Mobile IP environment 2 includes the internet (or a WAN) 4 over which a Mobile Node 6 can communicate remotely via mediation by a Home Agent 8 and may also include a Foreign Agent 10. In the absence of a Foreign Agent in a Mobile IPv4 environment, or in a Mobile IPv6 environment in which a Foreign Agent is not implemented, the Mobile Node 6 can obtain a topologically correct IP address (i.e., collocated IP address) and register this IP address with the Home Agent. (In a Mobile IPv6 environment, this is accomplished via an Access Router rather than a Foreign Agent.) Typically, the Home Agent and Foreign Agent are routers or other network connection devices performing appropriate Mobile IP functions as implemented by software, hardware, and/or firmware. A particular Mobile Node (e.g., a laptop computer) plugged into its home network segment connects with the internet through its designated Home Agent. When the Mobile Node roams, it communicates via the internet through an available Foreign Agent. Presumably, there are many Foreign Agents available at geographically disparate locations to allow wide spread internet connection via the Mobile IP protocol. Note that it is also possible for the Mobile Node to register directly with its Home Agent.
As shown in FIG. 1, Mobile Node 6 normally resides on (or is “based at”) a network segment 12 which allows its network entities to communicate over the internet 4 through Home Agent 8 (an appropriately configured router denoted R2). Note that Home Agent 8 need not directly connect to the internet. For example, as shown in FIG. 1, it may be connected through another router (a router R1 in this case). Router R1 may, in turn, connect one or more other routers (e.g., a router R3) with the internet.
Now, suppose that Mobile Node 6 is removed from its home base network segment 12 and roams to a remote network segment 14. Network segment 14 may include various other nodes such as a PC 16. The nodes on network segment 14 communicate with the internet through a router which doubles as Foreign Agent 10. Mobile Node 6 may identify Foreign Agent 10 through various solicitations and advertisements which form part of the Mobile IP protocol. When Mobile Node 6 engages with network segment 14, Foreign Agent 10 relays a registration request to Home Agent 8 (as indicated by the dotted line “Registration”). The Home and Foreign Agents may then negotiate the conditions of the Mobile Node's attachment to Foreign Agent 10. For example, the attachment may be limited to a period of time, such as two hours. When the negotiation is successfully completed, Home Agent 8 updates an internal “mobility binding table” which specifies the care-of address (e.g., a collocated care-of address or the Foreign Agent's IP address) in association with the identity of Mobile Node 6. Further, the Foreign Agent 10 updates an internal “visitor table” which specifies the Mobile Node address, Home Agent address, etc. In effect, the Mobile Node's home base IP address (associated with segment 12) has been shifted to the Foreign Agent's IP address (associated with segment 14).
Now, suppose that Mobile Node 6 wishes to send a message to a Correspondent Node 18 from its new location. In Mobile IPv4, a message from the Mobile Node is then packetized and forwarded through Foreign Agent 10 over the internet 4 and to Correspondent Node 18 (as indicated by the dotted line “packet from MN”) according to a standard internet protocol. If Correspondent Node 18 wishes to send a message to Mobile Node—whether in reply to a message from the Mobile Node or for any other reason—it addresses that message to the IP address of Mobile Node 6 on sub-network 12. The packets of that message are then forwarded over the internet 4 and to router R1 and ultimately to Home Agent 8 as indicated by the dotted line (“packet to MN(1)”). From its mobility binding table, Home Agent 8 recognizes that Mobile Node 6 is no longer attached to network segment 12. It then encapsulates the packets from Correspondent Node 18 (which are addressed to Mobile Node 6 on network segment 12) according to a Mobile IP protocol and forwards these encapsulated packets to a “care of” address for Mobile Node 6 as shown by the dotted line (“packet to MN(2)”). The care-of address may be, for example, the IP address of Foreign Agent 10. Foreign Agent 10 then strips the encapsulation and forwards the message to Mobile Node 6 on sub-network 14. The packet forwarding mechanism implemented by the Home and Foreign Agents is often referred to as “tunneling.” In the absence of a Foreign Agent, packets are tunneled directly to the Mobile Node 6 collocated care-of address.
In the absence of a Foreign Agent, when a Mobile Node roams to a new location, it is assigned a collocated care-of address. The collocated care-of address is an IP address that is local to the network and subnet in which the Mobile Node has roamed. This IP address is generally allocated by a server 20 such as a DHCP server from a pool of available IP addresses. Since network users are highly mobile, the networks and subnets to which they are connected change frequently. When an IP address is allocated to a user, the IP address is typically allocated for a period of time. The period of time for which the IP address is allocated is typically referred to as a “lease time.” The DHCP server is typically configured to allocate an IP address for a particular period of time. For example, the lease time could be a week. Of course, in a highly mobile environment, a user might connect to a particular network for only a few hours or minutes before moving to a new network. Unfortunately, the IP address that has been allocated and assigned to that user is not re-usable within the network until the lease time expires, at which time the address is returned to the pool of available IP addresses.
In addition, IP addresses may also be allocated as home addresses. For instance, a home address may be allocated by a Home Agent to a Mobile Node. One system for dynamically allocating home addresses to Mobile Nodes is described in Attorney Docket No. CISCP078, U.S. Pat. No. 6,501,746, entitled “Mobile IP Dynamic Home Address Resolution,” by Kent Leung, filed on Jan. 8, 1999 and issued on Dec. 31, 2002, which is incorporated herein for all purposes. While IP addresses are allocated as home addresses, there fails to be a mechanism for releasing those IP addresses when no longer in use.
In view of the above, it would be beneficial if IP address use could be optimized in a Mobile IP environment.